Alkylene carbonates, such as ethylene carbonate and propylene carbonate are widely used as solvents and diluents in industrial processes. They are regularly used as raw materials for commercial products such as cosmetics and pharmaceuticals. Alkylene carbonates can also be used as intermediates in the preparation of alkylene glycols from alkylene oxides.
Alkylene carbonates are produced commercially by the reaction of carbon dioxide with the appropriate alkylene oxide. In the art, ionic halides, such as quaternary ammonium halides, quaternary phosphonium halides and metal halides, are frequently proposed as catalysts for this reaction.
According to JP-A-57,106,631, the preparation of alkylene carbonate as an intermediate in the two-step preparation of alkylene glycol can occur by the reaction of an alkylene oxide with carbon dioxide in the presence of an alkali metal halide.
U.S. Pat. No. 4,314,945 is directed to the preparation of an alkylene carbonate by reaction of the corresponding alkylene oxide with carbon dioxide in the presence of a catalyst characterized by the formula M+A−, wherein M is potassium and A is iodine or M is a quaternary ammonium cation (R1R2R3R4N+) and A is either bromine, chlorine or iodine. The reaction is carried out in alkylene carbonate.
U.S. Pat. No. 4,786,741 is directed to the reaction of alkylene oxide with carbon dioxide in the presence of a catalytic composition and water. Catalytic compositions listed include organic quaternary ammonium halides, organic quaternary phosphonium halides, organic sulphonium halides and organic antimony halides.
JP-A-59,013,741 teaches a method for producing ethylene glycol from ethylene oxide via ethylene carbonate. The reaction of ethylene oxide with carbon dioxide to form ethylene carbonate is catalysed with a quaternary phosphonium halide.
The use of a combination of an alkali metal halide and manganese halide as a catalyst for the preparation of alkylene carbonate from alkylene oxide has been described in U.S. Pat. No. 6,160,130. Lead and indium halides in combination with an alkali metal halide are taught as suitable catalysts for this reaction in U.S. Pat. No. 6,156,909.
Kim et al. have described the use of zinc halides in combination with various other compounds as effective catalysts for the carboxylation of alkylene oxide. In J. Catal. (2003) 220, 44-46, catalysts formed by the reaction of 1-alkyl-3-methylimidazolium halides with zinc halides is described. Catalysts comprising zinc halides coordinated with pyridines are described in Angew. Chem. Int. Ed. (2000) 39(22), 4096-4098, Chem. Eur. J. (2003) 9(3), 678-686 and U.S. Pat. No. 6,399,536.
Mixtures of zinc halides and alkylammonium iodides as catalysts for the conversion of alkylene oxide to alkylene carbonate are taught in Chem. Ber. (1986) 119, 1090-1094.
Homogeneous catalysts composed of one of a number of metal salts in combination with a halide selected from the group of alkali metal halides, alkaline earth metal halides, quaternary ammonium, quaternary phosphonium, quaternary arsenonium, quaternary stibonium halides and ternary sulphonium halides have been described for use in the conversion of alkylene oxide to alkylene carbonate in U.S. Pat. No. 5,218,135 and U.S. Pat. No. 5,391,767.
Significant progress has been reported in the development of homogeneous catalysts for the carboxylation of alkylene oxide. However, the use of such catalysts leads to further process steps in order to purify the desired product. Furthermore, as described by Kim et al. in Appl. Catal., A (2005) 288, 48-52, separation of alkylene carbonate from homogeneous catalysts by distillation can result in severe decomposition of the alkylene carbonate.
Heterogeneous catalysts for the carboxylation of propylene oxide to propylene carbonate, consisting of quaternary phosphonium halides immobilized on silica, were reported by Takahashi, et al. in Chem. Commun. (2006) 1664-1666. The carboxylation of epoxides using catalysts consisting of quaternary ammonium and quaternary phosphonium salts bound to insoluble polystyrene beads was reported by Nishikubo, et al. in J. Polym. Sci., Part A: Polym. Chem., (1993) 31, 939-947.
A solid-supported zinc halide, wherein the solid support is poly(4-vinylpyridine) is described by Kim et al. in J. Catal. (2002) 205, 226-229. However, this system is described as having reduced activity in comparison to the equivalent homogeneous system.
A solid-supported system based on zinc halide, wherein the solid support is either poly(4-vinylpyridine) or chitosan is described by Xiao et al. in Appl. Catal., A (2005) 279, 125-129. A homogeneous 1-butyl-3-methylimidazolium bromide co-catalyst must also be used in this system.
There still remains a need for a catalyst system for the conversion of alkylene oxides to alkylene carbonates which allows easy purification of the desired product without decomposition of that desired product and still demonstrates high levels of selectivity and activity.